speaker grilles

Here is question I was asked the other day. How much listening improvement is gained from removing the speaker grilles? Do manufactures assume you will listen will the grilles off and supply them only for looks and keeping dust off while not listening?
Some speakers are in fact voiced to be heard with the grills on but I would imagine that most high end speakers are not. Listening with the grills on will muffle the tweeter to some degree, the mid range to a lesser degree. The grill frame could also cause reflections and distort the sound. However with some electronics which are inherrently bright this might not be an all together bad thing. The amount of muffling will depending on the type of grill cloth. But with a well matched system, grills off will allow you to hear all of the sound that your speaker (and electronics) can resolve. For me thats the preferred way. I put the grills back on when company comes. :-)
paradigm is one company that I can think of that recommends listening with the grills on. Like newbee said, that is how they are design. Most other companies recommend removing the grills during listening.
I think all speaker sound better with grills off. Theil is another that says they designed with the grills in place. I think it is a sales pitch. It's like "hey we build such a great speaker and design such a great cabinet that our speakers sound better through fabric"?? Take 'em off, take 'em all off!
I was told by my dealer that JM Lab voices all its speakers for use with the grill on. In practice though, when I took the grills off, the sound opened up, detail improved, and the soundstage widened by about 6 feet! So I don't buy that claim.

I think manufacturers say these things so that people evaluate a product's aesthetics with the grill on rather than with the grill off. Most non-audiophiles prefer looking at grills rather than drivers. The WAF factor, maybe.
My JM Labs also sound better with the grills off...although my soundstage doesn't grow 6 feet! Dag nabbit.
i myself try to avoid pulling the grills off of my speakers as much as possible,from what ive heard in my systems any sonic improvements are so small(if any) that its not worth the risk of damage from constantly handeling them.

My concern too Bigjoe, I started thinking about this after a friend told me he had just broken the post off a speaker grill, he always listens with grilles off. But I will probably a least give it a listen with my grilles off. I asked my manufacturer Spendor, they said their fabric does not defract high frequency, but said some owners prefer the grilles off and encouraged me to try it both ways. I posted the same question on Audio Assylum and got a good response from 'Bobby'.
I convinced myself it isn't worth taking the grills off all the time. I don't want to leave them off because the sun shines on them in the morning....